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Handbook of instrumental techniques from CCiTUB
Paleomagnetism: principles and
applications
Elisabet Beamud
Laboratori de Paleomagnetisme, CCiTUB, Universitat de Barcelona. Institut de
Ciencies de la Terra “Jaume Almera”, Solé i Sabarés s/n. 08028 Barcelona.
Spain.
email: betbeamud@ccit.ub.edu
Abstract. This article summarizes the basic principles of Paleomagnetism, with
examples of applications in geology that illustrate the capabilities of the
technique.
Paleomagnetism: principles and applications
1. Introduction
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Paleomagnetism is the study of the record of the Earth’s magnetic field in rocks. The Earth’s
magnetic field has its origin in the convection of the iron rich material of the outer core of the
Earth. At any point of the earth surface, the magnetic field (F) can be defined by two angles (Figure
1). Declination (Dec) is the angle between the horizontal component of F and the geographic north,
ranging from 0 to 360 º and defined as positive clockwise; inclination (Inc) is the vertical angle or
the angle between F and the horizontal, ranging from -90 to 90 º, positive downwards.
Figure 1 – Description of the direction of the
magnetic field (F). Inclination (Inc) is the vertical
angle (= dip) between the horizontal and F;
declination (Dec), is the azimuthal angle between
the horizontal component of F (H) and the
geographic north.
At the Earth’s surface, the geomagnetic field can be approximated to the field created by a
geocentric axial dipole (GAD), a dipole placed at the center of the earth and aligned with the
Earth’s rotation axis (Figure 2). The GAD model only accounts for about the 90% of the Earth’s
surface field, the rest would be the sum of non geocentric dipolar fields, fields of non-dipolar origin
and of external origin. In the Earth’s surface, magnitude and direction of the geomagnetic field
changes with time, ranging from milliseconds, hours, or days (pulsations or short-term fluctuations,
daily magnetic variations, or magnetic storms) to centuries, thousands of years, or million of years
(secular variations, magnetic excursions, and polarity reversals). Changes with periods between 1
year and 105 years constitute which is known as Geomagnetic Secular Variation. Among the longer
term variations of the geomagnetic field the polarity reversals, with durations ranging from
thousands of years to million years, are the switch of north and south magnetic poles. The present
configuration of the dipole field with the north magnetic pole close to the north geographic pole is
considered a normal polarity interval, while opposite configuration is considered a reversed
polarity interval (Figure 2). Intervals of geological time having a constant geomagnetic field
polarity delimitated by reversals are defined as polarity chrons. The magnetic reversals are longperiod processes (longer than 105 years) which do not occur with a simple periodicity but instead
appear to occur randomly in time. The duration of a polarity transition is imperfectly known but it
is probably in the order of 103 to 104 years, fast enough to be considered globally synchronous on
geologic time scales.
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Paleomagnetism: principles and applications
The pattern of polarity reversals providing a unique global “bar-code” recorded in Earth’s rock
of different ages constitutes the geomagnetic polarity time scale (GPTS) (Figure 2). For the
construction of the GPTS geophysicists fundamentally rely on the marine magnetic anomaly record
and also on the magnetostratigraphic record, both containing biostratigraphical constraints. Oceanic
surveys carried out during the 1950’s and 1960’s and equipped with shipboard magnetometers
found linear magnetic anomalies parallel to the mid-oceanic ridges. These anomalies resulted from
the remanent magnetization of the oceanic crust, acquired during the process of seafloor spreading.
The spreading process results in the magnetization of the crust in alternating normal and reverse
polarity. The template of magnetic anomaly patterns from the ocean floor has remained essential
for constructing the GPTS from Early Cretaceous onward (ca. 124.5 Ma to 0 Ma). The initial
assumption of periodic behavior (Cox et al, 1963) was soon abandoned as new data became
available. The first modern GPTS based on marine magnetic anomaly patterns was established by
Heirtzler et al. (1968). Subsequent revisions by Labreque et al. (1977), Berggren et al. (1985),
Cande and Kent (1992) showed improved age control and increased resolution. A major
breakthrough came with the astronomical polarity time scale (APTS) in which every individual
reversal is accurately dated (e. g. Hilgen et al., 1997). Because there is an excellent first-order
reversal chronology for most of the Mesozoic and all of the Cenozoic and because these reversals
occur randomly in time, sequences of reversals can be used for establishing the age of the rocks.
Figure 2 – Schematic representation of the geomagnetic field of a geocentric axial dipole. During
normal polarity of the field the average magnetic north pole is at the geographic north pole, and a
compass aligns along magnetic field lines. During normal polarity, the inclination is positive
(downward directed) in the northern hemisphere and negative (upward directed) in the southern
hemisphere. Conversely, during reversed polarity, the compass needle points south, and the
inclination is negative in the northern and positive in the southern hemisphere. In the geomagnetic
polarity time scale (column in the middle), periods of normal polarity are conventionally
represented by black intervals, reversed intervals by white intervals. From Langereis et al (2010)
The study of the ancient Earth’s magnetic field is possible because rocks contain ferromagnetic
minerals in their internal structure. During the rock-forming processes these minerals statistically
align with the ambient field, and are subsequently “locked in” the rock system, thus preserving the
direction of the field as a natural remanent magnetization (NRM). Principal minerals contributing
to the NRM are usually iron oxides such as magnetite, hematite, and maghemite, iron hydroxides
(goethite), or iron sulphides including pyrrhotite and greigite. Other common minerals such as
phyllosilicates (e.g., ilmenite), pyroxenes, and amphiboles can significantly contribute to the
induced magnetization. However, they do not have the capability to carry a NRM.
As currently practiced, Paleomagnetism can be applied to age determination, stratigraphy,
tectonics, polar wander, magnetic anomaly interpretation, paleoclimatology, as well as studies of
the evolution and history of the Earth’s magnetic field.
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Paleomagnetism: principles and applications
2. Methodology
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2.1. Sampling
Samples are drilled in the field either with an electric or gasoline power drill cooled with water
(Fig. 3). As the earth’s magnetic field is a vector, samples need to be oriented in situ to get a
geographic unambiguous position and orientation in order to provide a reference system to which
refer the data obtained during the laboratory analyses. Therefore, samples are oriented in the field
by a magnetic compass coupled to an orienting device with inclinometer (Fig. 4). The length of the
cores is variable but it is desirable that 2 samples of 10 cm3 can be obtained from each core.
Sampling strategy may differ according to the main focus of the study. However, in any case,
geological information such as bedding direction, presence of faults or fold axes orientations will
be collected during the field work. For magnetostratigraphic purposes, 1 or 2 samples per level will
be obtained at a certain stratigraphic spacing (depending on the total thickness of the stratigraphic
section to be studied and the number of magnetozones expected to be identified within the section).
Sampling focused on tectonic studies, such as identification of vertical-axis rotations within
orogenic belts, will consist in the drilling of about 10 cores distributed along the site with an
spacing enough to average the geomagnetic secular variation. Several sites distributed all along the
study area will be drilled to get a regional pattern of tectonic rotations. In the case of
archaeomagnetic studies, about 10-15 samples will be drilled in a random distribution within the
archaeological structure and, whenever possible, they will be oriented with a sun compass in order
to minimize the perturbation in the magnetic compass caused by the intense magnetization of the
archaeological material.
Figure 3 – Paleomagnetic sampling
Figure 4 – In situ orientation of drilled cores
2.2. Instrumentation and Measurement in the Laboratory
Paleomagnetism is mainly based on the measurement of the NRM of geological material, which is
the fossil magnetism naturally present in a rock. The primary magnetization is the component of
the NRM that was acquired when the rock was formed and may represent all, part, or none of the
NRM. The primary magnetization may decay through time either partly or completely and
additional components, referred to as secondary magnetizations, may be added by several
processes. A major task in all paleomagnetic investigations is to identify and separate the magnetic
components in the NRM, using a range of demagnetization and analysis procedures.
The main laboratory analysis consists in progressive demagnetization of the samples and the
subsequent measurement of the NRM in order to identify and isolate the different paleomagnetic
components recorded during the rock-geological history. Progressive demagnetization of the
samples can be achieved by two different treatments, thermal (TH) or by alternating fields (AF).
During demagnetization experiments, samples are subjected to stepwise increasing values of
temperature or alternating field in a zero magnetic field (field-free) space. The residual
magnetisation is measured after each demagnetization step and the resulting changes in direction
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Paleomagnetism: principles and applications
and intensity are displayed and analysed in order to reconstruct the complete component structure
of the NRM. Measurements are made of components (Mx, My and Mz) of magnetic moment of the
specimen (in sample coordinates). This usually entails multiple measurements of each component,
allowing evaluation of homogeneity of NRM in the specimen and a measure of signal-to-noise
ratio. Data are then fed into a computer containing orientation data for the sample and calculation
of the best-fit direction of NRM in sample, geographic and stratigraphic coordinates is carried out.
2.3. Data analysis
Vector directions are described in terms of declination (D) and inclination (I). After the complete
demagnetization of the samples, these directions are displayed with a projection that depicts threedimension information on a two-dimension environment. Although projections on a sphere are
commonly used, the most common representation of results of progressive demagnetization are
vector end point or orthogonal projection diagrams, also known as Zijderveld diagrams (Zijderveld,
1967). The power of this display is the ability to display directional and intensity information on a
single diagram by projecting the vector onto two orthogonal planes (Figure 5). Magnetic
components are then extracted from the Zijderveld diagrams using least-square analysis
(Kirschvink 1980), and the most stable and consistent component that can be isolated is referred to
as the characteristic remanent magnetisation (ChRM). This ChRM is further investigated to
establish if it represents a record of the geomagnetic field at, or close to, the time of rock formation,
or a secondary magnetisation acquired later in geologic history by post-depositional processes.
With this purpose, several stability tests can be performed on the obtained ChRM, being the most
common ones:
-
-
-
Consistency test: a ChRM is considered primary in origin when it defines a sequence of
polarity reversals laterally traceable by independent means (e.g., lithostratigraphy) between
distant sections from different parts of the basin.
Reversal test: the observation of ChRM directions with different polarity and, in particular,
the occurrence of antiparallel (within statistical error) directions is taken as a strong
indication for the primary origin of that ChRM.
Fold test: if the ChRM directions from differently tilted beds converge after correction for
the dip of the strata, this remanence was acquired before tilting. Strictly speaking, this fold
test does not directly prove a primary origin of this component, but only that it dates from
before tilting.
Figure 5 - Examples of Zijderveld diagrams. Solid (open) circles correspond to projection onto the
horizontal (vertical) planes. a: sample of normal polarity (northward declination and positive
inclination); b: sample with a low temperature normal polarity component, defined between 30º
and 300 ºC, parallel to the present-day magnetic field and a ChRM of reversed polarity, defined
from 300º to 670 ºC (southward declination and negative inclination)
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Paleomagnetism: principles and applications
3. Examples of applications
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3.1. Magnetostratigraphy of Can Mata (Hostalets de Pierola, Vallès-Penedès Basin, NE Spain)
As mentioned above, magnetostratigraphy is based on the fact that the earth’s magnetic field
polarity changes through time at a non-constant frequency and on the fact that sediments have the
capability to record and retain the magnetic field present at the moment of their formation.
Therefore, if we study a sedimentary succession long and continuous enough we will be able to
identify a reversals characteristic pattern which will allow us to establish a correlation to the
Geomagnetic Polarity Time Scale (GPTS). The example presented corresponds to the
magnetostratigraphic dating of the Middle Miocene sediments of Can Mata in the Hostalets de
Pierola area, a sedimentary succession containing many fossil remains, being the main one among
them the remnants of Pierolapithecus catalanicus and the hominoid Dryopithecus fontani. The
magnetostratigraphic study was carried out in order to provide the mammal bearing sediments of
Can Mata a robust absolute chronology. This paleomagnetic study was based on the analysis of 369
samples uniformly distributed along 460 m composite section. The correlation of the local
magnetostratigraphy to the GPTS (Gradstein et al, 2004) (Figure 6) establishes the age of the
sedimentary succession in Upper Aragonian-Vallesian (13 to around 10.5 Ma), and the estimated
age of Dryopithecus fontani is established in 11.8 Ma (Moyà-Solà, et al, 2009).
Figure 6 - Composite
magnetic polarity stratigraphy
of Can Mata and correlation
with GPTS2004 (Gradstein et
al, 2004).
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Paleomagnetism: principles and applications
3.2. Archaeomagnetic dating of combustion structures
Archaeomagnetic studies in Spain have undergone a significant progress during the last few years
and a reference curve of the directional variation of the geomagnetic field over the past two
millennia is now available for the Iberian Peninsula (Gómez-Paccard et al, 2006). These recent
developments have made archaeomagnetism a straightforward dating tool for Spain and Portugal.
In this example, we illustrate how this secular variation curve (SVC) can be used to date the last
use of burnt structures from Spain. A kiln of the archaeological site of Can Xammar (Figure 7)
(Mataró) of Mediaeval times has been studied and archaeomagnetically dated (Gómez-Paccard and
Beamud, 2008). The directions of the characteristic remanent magnetization of the structure have
been obtained from classical thermal and alternating field (AF) demagnetization procedures on the
collected samples, and a mean direction for the combustion structure has been obtained (Figure 8).
These directional results have been compared with the new reference curve for Iberia, providing
archaeomagnetic dates for the last use of the kiln between 1492 and 1609, consistent with the
archaeological evidences available (Figure 9).
Figure 7 - Combustion structure of the Can Xammar archaeological site and location of the
samples. Modified from Gómez-Paccard and Beamud (2008)
Figure 8 - Mean direction for the Can Xammar kiln. Modified from Gómez-Paccard and Beamud
(2008)
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Paleomagnetism: principles and applications
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Figure 9 - Comparison of the obtained
direction with the SVC for the Iberian
Peninsula and possible dates for the
last use of the combustion structure.
The probability density functions
(PDF) for Declination and Inclination
are combined to obtain the possible
dates at 95% probability for the last
usage of the structure. In the example
presented, the age of the abandonment
of the structure is established to have
occurred between 1492 and 1609.
Modified from Gómez-Paccard and
Beamud (2008)
3.3. Tectonic rotations in the south-central Pyrenees during theTertiary
The ability of rocks to record the direction of the ancient magnetic field provides the key
quantitative information for unravelling tectonic deformation in orogenic belts. The orogenic
environment, however, causes specific problems to the analysis of the magnetism in rocks due to
the effects of strong deformation and temperature increase. Rocks in this environment are more
likely to have their primary magnetism partially or completely overprinted by magnetizations
linked to the deformation. Partially set against this disadvantage, the deformed nature of the rocks
means that the paleomagnetic fold test is more readily applied to constrain the geological ages of
the magnetizations than would be possible in less deformed settings. Paleomagnetic analysis
assumes that the mean direction of magnetization recovered from the rock-unit is a record of the
time averaged dipole field source at the time of magnetization. The cumulative deformation can
then be resolved by comparing this direction with the predicted direction at the study location at the
time of magnetization. Usually, the difference is the resultant of more than one episode of
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Paleomagnetism: principles and applications
deformation and in favourable circumstances it can be decomposed using geological evidence or
from studying the paleomagnetism of rocks of different ages. The angular deformation is usually
considered in terms of differences of declination and inclination with respect to the reference
direction from the stable plate. The difference between the observed and expected directions is
described in terms of rotation and flattening. A difference in declination implies rotation (R) of a
crustal block about a quasi-vertical axis.
The Graus-Tremp piggy-back basin is an elongated trough formed during the emplacement of
the allochtonous units of the South Central Pyrenean Units. Previous studies in both the Mesozoic
basement and the Paleocene sediments of the Graus-Tremp basin have evidenced significant
vertical axis clockwise rotations (Figure 10). These rock units are unconformable overlain by the
syntectonic continental conglomerates of the Sierra de Sis and La Pobla de Segur, which were
recently dated as Bartonian to Priabonian on basis of magnetostratigraphic correlation (Beamud et
al, 2003). More recently, the syntectonic conglomerates of the Senterada basin have been dated as
Oligocene also based on magnetostratigraphy (Beamud et al, 2011). Paleomagnetic data from these
units provide constraints on the timing and spatial distribution of the tectonic rotations in the
Graus-Tremp Basin. No significant rotations during the Tertiary are observed near the Noguera
Pallaresa River. Further west, between the Esera and the Ribagorçana rivers, the lower Eocene
materials record up to 30º of clockwise rotation. The lack of significant rotation in the overlying
Bartonian sediments of the Sierra de Sis constrains this rotation to the Ypresian-Lutetian. In the
Ainsa basin, about 30º of clockwise rotation is still recorded by Bartonian sediments. These data
suggest an east-west trending of the vertical axis rotations magnitude and timing.
Figure 10 - Distribution of the vertical-axis rotations within the Graus-Tremp piggy-back basin
during the Tertiary. Modified from Beamud et al (2004).
References
Beamud, E; Garcés, M; Cabrera, L; Muñoz, J.A; Almar, Y. (2003) A new Middle to Late
Eocene continental chronostratigraphy from NE Spain. Earth and Planetary Science
Letters 216 (4) 501-514
Beamud, E; Garcés, M; Muñoz, J.A; Cabrera, Ll; Almar, Y. (2004) Distribución de las
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rotaciones paleomagnéticas en la cuenca de Graus-Tremp durante el Terciario. Geotemas
6 (4) 283-285
Beamud, E., Muñoz, J.A., Fitzgerald, P.G., Baldwin, S.L., Garcés, M., Cabrera, L., Metcalf,
J.R. (2011) Magnetostratigraphy and detrital apatite fission track thermochronology in
syntectonic conglomerates: constraints on the exhumation of the South-Central Pyrenees.
Basin Research 23(3) 309-331
Berggren, W.A., Kent, D.V., Flynn, J.J., (1985) Cenozoic geochronology. Geological
Society of America Bulletin, 96, 1407-1418
Bentham, P.A. (1992) The tectono-stratigraphic development of the western oblique ramp of
the South-Central Pyrenean thrust system, Northern Spain. PhD Thesis, University of
California, 252 pp.
Cande, S. C., Kent, D.V., 1992. A new geomagnetic polarity time-scale for the Late
Cretaceous and Cenozoic. Journal of Geophysical Research 97, 13917–13951
Cox, A., Doell, R. R., Dalrymple, G. B., 1963. Geomagnetic polarity epochs and Pleistocene
geochronometry, Nature198, 1049–1051
Dinarés-Turell, J. (1992) Paleomagnetisme a les Unitats Sudpirinenques Superiors.
Implicacions estructurals. PhD Thesis, University of Barcelona
Gómez-Paccard, M., Chauvin, A., Lanos, P., McIntosh, G., Osete, M.L., Catanzariti, G.,
Ruiz-Martínez, V.C., Núñez, J.I., (2006). The first archaeomagnetic secular variation
curve for the Iberian Peninsula. Comparison with other data from Western Europe and
with global geomagnetic field models. Geochem. Geophys. Geosyst., 7, Q12001
Gómez-Paccard, M. & Beamud, E. (2008) Recent achievements in archaeomagnetic dating
in the Iberian Peninsula: Application to Roman and Mediaeval Spanish structures.
Journal of Archaeological Science, 35(5), 1389-1398
Gradstein, F.M., Ogg, J.G., Smith, A.G., (2004). A Geologic Time Scale 2004. Cambridge
University Press, Cambridge. 589pp
Heirtzler, J.R., Dickson, G.O., Herron, E.M., Pitman Iii, W.C., Lepichon, X., (1968). Marine
Magnetic Anomalies, Geomagnetic Field Reversals, and Motions of the Ocean Floor and
Continents. Journal of Geophysical Research, 73, 2119-2136
Hilgen, F. J., Krijgsman, W., Langereis, C. G., Lourens, L. J. (1997) Breakthrough made in
dating of the Geological Record, EOS, Transactions of the AGU 78, 285 & 288
Kirschvink, J.L., (1980). The least-squares line and plane and the analysis of paleomagnetic
data. Geophysical Journal of the Royal Astronomical Society, 62, 699-718
LaBreque, J. L., Kent, D.V., Cande S. C., 1977. Revised magnetic polarity time scale for
Late Cretaceous and Cenozoic time, Geology 5, 330–335.
Langereis, C.G., Krijgsman, W., Muttoni, G., Menning, M., (2010). Magnetostratigraphy concepts, definitions, and applications. Newsletter on Stratigraphy, 43, 207-233
Moyà-Solà, S; Köhler, M; Albà, D; Casanovas-Vilar, I; Galindo, J; Robles, J.M; Cabrera, Ll;
Garcés, M; Amécija, S; Beamud, E. - First partial face and upper dentition on the Middle
Miocene hominoid Dryopithecus fontani from Abocador de Can Mata Vallès-Penedès
Basin, Catalonia (Spain): taxonomic and phylogenetic implications. American Journal of
Physical Anthropology 139 (2), 126-145
Pascual, J.O; Parés, J.M; Langereis, C.G; Zijderveld, J.D.A. (1992) – Magnetostratigraphy
and rock magnetism of the Ilerdian stratotype at Tremp, Spain. Physics of the Earth and
Planetary Interiors, 74, 139-157.
Soriano, C., Beamud, E., Garcés, M. (2008) Magma flow in dikes from rift zones of the
basaltic shield of Tenerife, Canary Islands: implications for the emplacement of buoyant
magma. Journal of Volcanology and Geothermal Research, 173(1-2), 55-68
Zijderveld, J.D.A. (1967) AC demagnetization of rocks: Analysis of results, In: Methods in
Paleomagnetism, ed. D.W. Collinson, K.M. Creer and S.K. Runcorn, Elsevier,
Amsterdam, pp.254-286
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